This invention relates to a particulate fertilizer composition prepared by the reaction of urea and formaldehyde.
Urea-formaldehyde condensation products are widely used as slow or controlled release nitrogen fertilizers. The condensation products contain methylene urea polymers of varying chain length. The higher methylene urea polymers have limited solubility in soil solution and hence serve to prolong the release of nitrogen. The method of nitrogen release is normally thought to be by microbial decomposition. Most of the literature relating to such products has emphasized the importance of the longer chain water insoluble polymers which have been considered safer and desirable for slow release. The products have traditionally been characterized in terms of the water insolubility of their nitrogen fractions. The 1980 standards of the Association of American Plant Food Control Officials (AAPFCO), for example, requires that any fertilizer labeled as urea-formaldehyde, or ureaform, must have at least 60% of its nitrogen in water-insoluble form with an activity index of at least 40 (Official Publication, AAPFCO, No. 33, Rules N-24, N-25, 1980). Activity Index (AI) is the percent of the cold water insoluble nitrogen (CWIN) which is soluble in hot water: ##EQU1##
The longer chain water insoluble polymers are, in fact, much less agronomically active than the shorter chain polymers. Generally, the efficiency of plant utilization decreases as the methylene urea polymer chain increases. It has been known for some time that a portion of the cold water insoluble nitrogen fraction (CWIN) has a very low mineralization rate and is very inefficient in terms of its practical use to a plant. This inefficient fraction is the hot water insoluble nitrogen (HWIN). Accordingly, the official definition of AAPFCO limits the allowable presence of this HWIN fraction. However, even with this limitation, a substantial portion of the nitrogen (36%-60%) may still be present in this very inefficient hot water insoluble form. Attempts have been made to increase the efficiency of the product by mixing the urea-formaldehyde reaction products with soluble nitrogen sources such as urea. This has the effect of decreasing the HWIN fraction, increasing the available nitrogen and providing "quick release" properties to the mixture. However, it also adds substantially to the "burn" or phytotoxic potential of the mixture. It is important to note that urea is present in substantially all urea-formaldehyde fertilizer compositions from incomplete or equilibrium reactions during polymerization. Such unreacted urea has the same agronomic response and tolerance as commercially available urea which is deliberately added.
The manufacture of slow release urea-formaldehyde reaction products for fertilizer applications requires considerable skill to produce the proper degree of polymerization required to achieve the desired fertilizer characteristics. Normally, they are prepared by first reacting urea and formaldehyde at elevated temperatures in an alkaline solution to produce methylol ureas. The reaction mixture is then acidified which causes the methylol ureas to polymerize rapidly to form methylene urea polymers of varying chain length.
U.S. Pat. No. 4,089,899 to Greidinger et al discloses what is there described as a controlled reaction system for preparing slow release ureaform compounds with a low average degree of polymerization. The Greidinger et al process involves the reaction of urea and formaldehyde in the presence of an acid catalyst for extended reaction times at low temperatures. While the patent discloses urea-formaldehyde compositions containing polymers having a somewhat lower degree of polymerization than conventional ureaform fertilizers, the compositions still contain large proportions of longer chain polymers. Moreover, the Greidinger et al process is incapable of producing compositions having a significantly lower degree of polymerization than those there shown.
U.S. Pat. No. 3,677,736 to Formaini discloses a multi-stage process for producing a liquid fertilizer suspension of ureaform. Broadly, the process involves the reaction of urea and formaldehyde in the presence of ammonia at an alkaline pH followed by acidification. The final product is stated to be a liquid fertilizer containing ureaform having a relatively high activity index, that is, a relatively small amount of the hot water insoluble (HWIN) fraction.